1. Field of the Invention
This invention relates to a circuit module for a phased array radar.
2. Discussion of Prior Art
Phased array radars have been under development for over twenty years to overcome the problems of mechanically scanned radars. An example of the latter typically employs a reflecting dish antenna rotated by a servomotor. Both antenna and servomotor are costly and cumbersome; the maximum beam scan rate is limited by the inertia and limited motor power of the antenna assembly.
In a phased array radar system, beamsteering or beamforming, ie control of the radar transmission or reception direction, is electronic instead of mechanical. Such a system consists of an array of antenna elements each connected to radar signal generation and reception circuitry. Each antenna element radiates when supplied with radar frequency (RF) power, and responds to incident radiation of appropriate frequency by producing a received signal. Received signals are downconverted to intermediate frequency (IF) by mixing with a local oscillator (LO) signal; ie, conventional superheterodyne detection is employed. In transmission, the output radar beam direction is controlled by the phase relationship between the RF drive signals to individual antenna elements in the array. If the drive signals are all in phase with one another, the output beam direction is perpendicular to the phased array ("on boresight") in the case of a planar system. If the drive signal phase varies linearly with antenna element position across the array, the output beam is inclined at an angle to the array boresight. Altering the rate at which phase varies with position alters the output beam inclination and provides the received signal phase as a function of array position. This may be achieved by varying the LO phase across the array, or by inserting differing delays into received signal paths. In both reception and transmission, control of signal phase at each individual array element is a prerequisite to a viable phased array radar. Conventional phase shifters employ switched lengths of transmission line, ferrite devices or switched networks of inductors and capacitors. They are bulky, costly and imperfect.
The development of phased array radars has been inhibited by the conflicting requirements of phase control and the need to increase radar frequency and power. It is desirable to employ a frequency which is as high as possible in order to reduce antenna size for a given angular resolution, which is inversely proportional to frequency. However, as the radar frequency increases, the cost of radar signal sources increases and power available is reduced. Moreover, the cost of electronic components suitable for the higher frequencies increases greatly and their availability deteriorates. Silicon integrated circuits for example are unsuitable for use at GHz frequencies at which state of the art phased arrays are required to operate. This has led to the development of GaAs monolithic microwave integrated circuits (MMIC) for operation at GHz frequencies.
The present design philosophy for phased arrays is that each antenna element be furnished with a respective transmit/receive circuit module, as described by Wisseman et al in Microwave Journal, September 1987 pages 167-172. This module incorporates a phase shifter for phase control, a transmission power amplifier and a low noise amplifier for received signals. It is for use at frequencies well above 1 GHz, and consists of a GaAs chip with dimensions 13.0.times.4.5.times.0.15 mm. The phase shifter is an analogue electronic circuit providing a choice of sixteen phase angles selectable by switches controlled by a 4 bit digital input. More than half of the area of the chip is devoted to the phase shifter, which is therefore responsible for a substantial proportion of the chip cost, failure rate and production faults. Chips of this kind are characterised by very high cost and low yield.
It is possible to avoid using individual phase shifters in a phased array operating in reception mode. This approach involves conventional frequency downconversion of individual antenna signals followed by digitisation and processing of the digital signals in a computer. The computer multiplies the digital signals by respective weighting factors, and sums the products so formed to produce a result corresponding to receive beamforming. There is however no equivalent of this procedure for the transmission mode.
In an attempt to ameliorate the problem of achieving controllable phase shifting, the technique of direct digital synthesis of analogue RF waveforms has been developed. This technique is described by R J Zavrel, in RF Design, March 1988, pages 27 to 31. It involves storing required analogue waveforms as set of digital numbers within a memory, and reading out the numbers in succession at a rate appropriate to the chosen frequency. A stream of digital numbers results which is fed to a digital to analogue converter. The converter output is the required waveform. Change of phase can be accomplished merely by change of start address. This approach is considerably more convenient than employing analogue phase shifter circuits. It also has the great advantage of sufficient flexibility to compensate for errors introduced by inaccuracies arising elsewhere in a phased array system. For example, a phase shift introduced by a single array module amplifier might be detected in a calibration operation. It would be compensated by a phase shift applied in that module by changing a start address.
However, direct digital synthesis suffers from the drawback that the highest frequencies that can be generated at present are more that a factor of ten below those required for a compact phased array radar. The latter requires transmission frequencies of several GHz or more, whereas digital synthesis is restricted to frequencies of hundreds of MHz. In consequence, and despite their disadvantages, analogue phase shifter circuits operating at radar frequencies are stiff employed in phased array transmission mode in state of the art devices such as that described by Wisseman et al.